Comparable和Comparator的理解

首先我们看以一下各自的JDK源码： 
在jdk1.8版本中Comparable接口仍然只有一个方法：compareTo ；

在jdk1.8版本中Comparator接口中增加了许多方法，接口也用了1.8才加进来的@FunctionalInterface注解。

Comparable的用法

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Employee implements Comparable {
    /**
     * id是根据进入公司的先后顺序排序
     */
    private int id;

    /**
     * 姓名
     */
    private String name;

    /**
     * 职位
     */
    private String position;

    public Employee(int id, String name, String position) {
        this.id = id;
        this.name = name;
        this.position = position;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPosition() {
        return position;
    }

    public void setPosition(String position) {
        this.position = position;
    }

    @Override
    public String toString() {
        return "id[" + id + "],name[" + name + "],position[" + position + "]";
    }

    @Override
    public int compareTo(Employee o) {
        if (this.getId() > o.getId()) {
            return 1;
        } else if (this.getId() < o.getId()) {
            return -1;
        } else {
            return 0;
        }
    }

    public static void main(String[] args) {
        List list = new ArrayList<>();

        list.add(new Employee(10001, "张三", "Boss"));
        list.add(new Employee(10006, "赵四", "Manager"));
        list.add(new Employee(10035, "王五", "Manager"));
        list.add(new Employee(10002, "李六", "staff"));
        list.add(new Employee(10005, "马牛", "staff"));
        // 按照id排序
        Collections.sort(list);
        // 如果要实现倒叙则调用下面的方法
        //Collections.reverse(list);
        for (Employee e : list) {
            System.out.println(e + "id:" + e.getId());
        }
    }
}

输出：

id[10001],name[张三],position[Boss]id:10001
id[10002],name[李六],position[staff]id:10002
id[10005],name[马牛],position[staff]id:10005
id[10006],name[赵四],position[Manager]id:10006
id[10035],name[王五],position[Manager]id:10035

Comparator的用法

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class EmployComparator implements Comparator {
    /**
     * id是根据进入公司的先后顺序排序
     */
    private int id;

    /**
     * 姓名
     */
    private String name;

    /**
     * 职位
     */
    private String position;

    public EmployComparator() {

    }

    public EmployComparator(int id, String name, String position) {
        this.id = id;
        this.name = name;
        this.position = position;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPosition() {
        return position;
    }

    public void setPosition(String position) {
        this.position = position;
    }

    @Override
    public int compare(EmployComparator o1, EmployComparator o2) {
        if (o1.getId() > o2.getId()) {
            return 1;
        } else if (o1.getId() < o2.getId()) {
            return -1;
        } else {
            return 0;
        }
    }

    public static void main(String[] args) {
        List list = new ArrayList();

        list.add(new EmployComparator(1001, "张三", "Boss"));
        list.add(new EmployComparator(1006, "赵四", "Manager"));
        list.add(new EmployComparator(10035, "王五", "Manager"));
        list.add(new EmployComparator(1002, "李六", "staff"));
        list.add(new EmployComparator(1005, "马牛", "staff"));
        // 按照id排序，也就是按照资历深浅排序,需要构造器
        Collections.sort(list, new EmployComparator());
        // 倒叙
        //Collections.reverse(list);
        for (EmployComparator e : list) {
            System.out.println(e + "id:" + e.getId());
        }
    }
}

output:

id[1001],name[张三],position[Boss]id:1001
id[1002],name[李六],position[staff]id:1002
id[1005],name[马牛],position[staff]id:1005
id[1006],name[赵四],position[Manager]id:1006
id[10035],name[王五],position[Manager]id:10035

未完待续...

2019.5.20更新：

// 修改Employee排序
Comparator comparator = new Comparator() {
    @Override
    public int compare(Employee o1, Employee o2) {
        if(o1.getName().hashCode() > o2.getName().hashCode()){
            return 1;
        }else if(o1.getName().hashCode() < o2.getName().hashCode()){
            return -1;
        }else {
            return 0;
        }
    }
};
// 指定比较器
Collections.sort(list,comparator);
for (Employee e : list) {
    System.out.println(e + "id:" + e.getId());
}

output：
d[10001],name[张三],position[Boss]id:10001
id[10002],name[李六],position[staff]id:10002
id[10035],name[王五],position[Manager]id:10035
id[10006],name[赵四],position[Manager]id:10006
id[10005],name[马牛],position[staff]id:10005

总结

• Comparable 自然排序。（实体类实现）
• Comparator 是定制排序。（无法修改实体类时，直接在调用方创建）